U.S. patent application number 12/079558 was filed with the patent office on 2009-06-18 for bipolar plate and process for producing a protective layer on a bipolar plate.
This patent application is currently assigned to ElringKlinger AG. Invention is credited to Thomas Kiefer.
Application Number | 20090155667 12/079558 |
Document ID | / |
Family ID | 39276023 |
Filed Date | 2009-06-18 |
United States Patent
Application |
20090155667 |
Kind Code |
A1 |
Kiefer; Thomas |
June 18, 2009 |
Bipolar plate and process for producing a protective layer on a
bipolar plate
Abstract
In order to provide a bipolar plate for a fuel cell unit,
wherein the bipolar plate comprises a support layer and a
protective layer, wherein the protective layer comprises an at
least binary oxide system with at least two different types of
metal cations, the protective layer of which reliably reduces
chromium evaporation even in long-term operation and which also
meets the other requirements set for a bipolar plate, it is
proposed that one type of metal cation of the oxide system of the
protective layer is Fe.
Inventors: |
Kiefer; Thomas; (Ettlingen,
DE) |
Correspondence
Address: |
Edward J. Timmer
P.O. Box 770
Richland
MI
49083
US
|
Assignee: |
ElringKlinger AG
|
Family ID: |
39276023 |
Appl. No.: |
12/079558 |
Filed: |
March 27, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
PCT/EP2007/011021 |
Dec 14, 2007 |
|
|
|
12079558 |
|
|
|
|
Current U.S.
Class: |
429/457 ;
427/115 |
Current CPC
Class: |
H01M 2008/1293 20130101;
Y02E 60/50 20130101; H01M 8/0228 20130101; H01M 8/0219
20130101 |
Class at
Publication: |
429/34 ;
427/115 |
International
Class: |
H01M 2/00 20060101
H01M002/00; B05D 5/12 20060101 B05D005/12 |
Claims
1. Bipolar plate for a fuel cell unit, wherein the bipolar plate
comprises a support layer and a protective layer, wherein the
protective layer comprises an at least binary oxide system with at
least two different types of metal cations, wherein one type of
metal cation of the oxide system of the protective layer is Fe.
2. Bipolar plate according to claim 1, wherein one type of metal
cation of the oxide system of the protective layer is Co or Cu.
3. Bipolar plate according to claim 1, wherein the oxide system of
the protective layer is an at least ternary oxide system with at
least three different types of metal cations.
4. Bipolar plate according to claim 1, wherein one type of metal
cation of the oxide system of the protective layer is Mn.
5. Bipolar plate according to claim 1, wherein the oxide system of
the protective layer comprises Mn, Co and Fe cations.
6. Bipolar plate according to claim 5, wherein the oxide system has
approximately the composition MnCo.sub.2-xFe.sub.xO.sub.4, where
0<x<1.
7. Bipolar plate according to claim 6, wherein the oxide system of
the protective layer has approximately the composition
MnCo.sub.1.9Fe.sub.0.1O.sub.4.
8. Bipolar plate according to claim 1, wherein the oxide system of
the protective layer comprises Mn, Cu and Fe cations.
9. Bipolar plate according to claim 1, wherein the coefficient of
thermal expansion .alpha. of the protective layer ranges from
approximately 1010.sup.-6K.sup.-1 to approximately
2010.sup.-6K.sup.-1.
10. Bipolar plate according to claim 1, wherein the specific
electrical conductivity .sigma. of the protective layer ranges from
approximately 0.01 S/cm to approximately 200 S/cm.
11. Process for producing a protective layer on a bipolar plate for
a fuel cell unit comprising the following process steps: applying a
layer of a protective layer starting material to a support layer of
the bipolar plate, wherein the protective layer starting material
comprises Fe cations; generating a reduced oxygen partial pressure;
increasing the temperature to a sintering temperature; subsequently
increasing the oxygen partial pressure; cooling the support layer
and the protective layer.
12. Process according to claim 11, wherein the support layer with
the starting material is not cooled between increasing the
temperature to sintering temperature and increasing the oxygen
partial pressure.
13. Process according to claim 11, wherein the starting material is
applied to the support layer using a wet-chemical method.
14. Process according to claim 13, wherein the starting material is
sprayed onto the support layer.
15. Process according to claim 13, wherein the starting material is
applied to the support layer using the screen-printing process.
16. Process according to claim 11, wherein the starting material
comprises at least three different types of metal cations.
17. Process according to claim 11, wherein the starting material
comprises Mn cations.
18. Process according to claim 11, wherein the starting material
comprises Co or Cu cations.
19. Process according to claim 11, wherein the starting material
comprises Ni cations.
20. Process according to claim 11, wherein the starting material
comprises Mn, Co and Fe cations.
21. Process according to claim 20, wherein the protective layer
produced has approximately the composition
MnCo.sub.2-xFe.sub.xO.sub.4, where 0<x<1.
22. Process according to claim 21, wherein the protective layer
produced has approximately the composition
MnCo.sub.1.9Fe.sub.0.1O.sub.4.
23. Process according to claim 11, wherein the starting material
comprises Mn, Cu and Fe cations.
Description
RELATED APPLICATION
[0001] This application is a continuation application of
PCT/EP2007/011021 filed Dec. 14, 2007, the entire specification of
which is incorporated herein by reference.
FIELD OF DISCLOSURE
[0002] The present invention relates to a bipolar plate for a fuel
cell unit, wherein the bipolar plate comprises a support layer and
a protective layer, wherein the protective layer comprises an at
least binary oxide system with at least two different types of
metal cations.
BACKGROUND
[0003] Since a fuel cell unit only has a low single cell voltage of
approximately 0.4 volts to approximately 1.2 volts (depending on
load), a series connection of a plurality of electrochemical cells
in a fuel cell stack is necessary, as a result of which the output
voltage is scaled to a range of interest from an applications
viewpoint. For this, the individual electrochemical cells are
connected by means of so-called bipolar plates (also referred to as
interconnectors).
[0004] Such a bipolar plate must meet the following requirements:
[0005] Distribution of the media (fuel gas and/or oxidising
agents). [0006] Adequate electrical conductivity, since within the
fuel cell unit the electrons generated on the hydrogen side (anode)
are directed through the bipolar plates in order to be available to
the air side (cathode) of the next electrochemical cell. To keep
the electrical losses low here, the material for the bipolar plates
must have an adequately high electrical conductivity. [0007]
Adequate corrosion resistance, since the typical operating
conditions of a fuel cell unit (operating temperature approximately
800.degree. C., oxidising/reducing atmosphere, moist air) have a
corrosive effect. For this reason, high requirements are set for
the corrosion resistance of the material of the bipolar plate.
[0008] Ferritic, chromium oxide-forming special steels are usually
used as material for the bipolar plates of high-temperature fuel
cells. One reason for this is the relatively good electrical
conductivity of the self-forming chromium oxide layer compared to
the insulating oxide layers, which are configured from other
high-temperature steels or alloys (e.g. aluminium oxide or silicon
oxide forming agents).
[0009] In the case of a temperature increase, chromium oxide forms
on the surface of a chromium oxide-forming special steel. Volatile
chromium compounds are formed from this chromium oxide under the
operating conditions of a fuel cell. In particular in the long-term
operation of the fuel cell unit, this "chromium evaporation"
results in a poisoning of the cathode, which causes a drastic
reduction in the current efficiency.
[0010] To prevent the chromium evaporation it has already been
proposed to dope specific elements (e.g. Mn, Ni, Co) into the steel
of the bipolar plate, which influence growth of the oxide layer and
convert the originally formed chromium oxide into a more chemically
stable form. While a minimisation of the chromium evaporation can
be achieved as a result of such alloy additions, no lasting
protection of the cathode is provided.
[0011] Moreover, it has already been proposed to coat the bipolar
plates with oxides or oxide mixtures (e.g. oxides of Mn, Co, Cu).
These layers are compacted as a result of solid-state diffusion by
a subsequent thermal treatment. Tests have shown that Mn, Fe and Cr
diffuse from the steel of the bipolar plate into such a protective
layer and thus assure compaction.
[0012] However, as a result of diffusion processes, the compacted
protective layer also contains chromium. Thus, the possibility of a
chromium evaporation that is associated with an increased
degradation of the cathode continues to exist.
[0013] A further bipolar plate with protective layer is known from
the article by Zhenguo Yang, Guanguang Xia and Jeffry W. Stevenson:
"Mn.sub.1.5Co.sub.1.5O.sub.4 Spinel Protection Layers on Ferritic
Stainless Steels for SOFC Interconnect Applications", published in
Electrochemical and Solid-State Letters, Volume 8 (3), pages A 168
to A 170 (2005).
[0014] In the production of the bipolar plate known from this
publication a protective layer is generated by a sintering process
of an oxide layer (Mn.sub.1.5Co.sub.1.5O.sub.4) applied by a
wet-chemical method in a reducing atmosphere. In this process a
paste containing a binder and oxides of the nominal composition
Mn.sub.1.5Co.sub.1.5O.sub.4 is applied to a steel material. The
sintering occurs in two separate thermal treatment steps. In the
first temperature cycle the lattice structure of the protective
layer is weakened by a reduction in the oxygen partial pressure, as
a result of which an improved sintering behaviour is achieved. In a
separate subsequent thermal treatment step the lattice structure is
fully oxidised again, i.e. the missing oxygen is incorporated into
the lattice again.
[0015] In this production process of a protective layer on a
bipolar plate the resulting microstructure of the protective layer
has numerous pores and cracks. Therefore, the protective layer
produced in this manner does not have any lasting chromium
retention power. Moreover, the physical material properties of the
protective layer, in particular its electrical conductivity and the
thermal expansion behaviour, are not optimally adapted to the
requirements set for a bipolar plate for a fuel cell unit.
SUMMARY OF THE INVENTION
[0016] The object forming the basis of the present invention is to
provide a bipolar plate of the aforementioned type, the protective
layer of which reliably also reduces chromium evaporation in
long-term operation and which also meets the other requirements set
for a bipolar plate.
[0017] This object is achieved according to the invention with a
bipolar plate with the features of the preamble of claim 1 in that
one type of metal cation of the oxide system of the protective
layer is Fe.
[0018] It has been found that the microstructure of the protective
layer is improved by the addition of iron (in particular with
respect to the reduction of cracks and pores). Therefore, iron has
a positive effect on the sintering behaviour of the protective
layer. The improved microstructure of the protective layer is an
indication that the protective layer is more defect-free. Since the
chromium diffusion is based, inter alia, on lattice defects, a
lasting chromium retention is assured by a more defect-free
protective layer.
[0019] Moreover, the addition of iron decreases the coefficient of
thermal expansion of the protective layer and therefore causes it
to be better adapted to the coefficients of thermal expansion of
the other components of the fuel cell unit. As a result, lower
mechanical stresses occur during a temperature cycle (heating to
operating temperature and cooling) of the fuel cell stack.
[0020] In addition, with a protective layer containing iron, a
second temperature cycle can be omitted in the sintering process
during production of the protective layer. If only a single
temperature cycle has to be run (instead of a first temperature
cycle for reduction of the protective layer and a second
temperature cycle for the subsequent oxidation), then this has a
positive effect on production costs and on the microstructure of
the protective layer.
[0021] The oxide system of the protective layer preferably has a
spinel structure.
[0022] It is favourable if the oxide system has at least one type
of metal cation, the oxide of which is more unstable than chromium
oxide (i.e. its stability limit in the Ellingham diagram is higher
than the stability limit of chromium oxide).
[0023] Moreover, it is favourable if the oxide system has at least
one type of metal cation, the oxide of which is more stable than
chromium oxide (i.e. its stability limit in the Ellingham diagram
is lower than the stability limit of chromium oxide).
[0024] Moreover, it is advantageous if a further type of metal
cation of the oxide system of the protective layer is Co or Cu.
[0025] As a result of the selection of an at least ternary oxide
system for the protective layer, an improved microstructure of the
protective layer and thus an improved reduction of chromium
diffusion through the protective layer and of chromium evaporation
can be achieved compared to protective layers comprising only
binary oxide systems. Therefore, it is advantageous if the oxide
system of the protective layer is an at least ternary oxide system
with at least three different types of metal cations.
[0026] In a preferred configuration of the invention it is provided
that one type of metal cation of the oxide system of the protective
layer is Mn.
[0027] In a preferred configuration of the invention it is provided
that the oxide system comprises Mn, Co and Fe cations.
[0028] It has proved particularly favourable if the oxide system
has approximately the composition MnCo.sub.2-xFe.sub.xO.sub.4,
where 0<x<1.
[0029] The oxide system with the approximate composition
MnCo.sub.1.9Fe.sub.0.1O.sub.4 has proved particularly
favourable.
[0030] Alternatively, it can also be provided that the oxide system
of the protective layer comprises Mn, Cu and Fe cations.
[0031] The composition of the protective layer of the bipolar plate
is preferably selected such that the coefficient of thermal
expansion .alpha. of the protective layer ranges from approximately
1010.sup.-6K.sup.-1 to approximately 2010.sup.-6K.sup.-1,
preferably from approximately 11.510.sup.-6K.sup.-1 to
approximately 13.510.sup.-6K.sup.-1. Such a coefficient of thermal
expansion is adapted particularly well to the thermal expansion
behaviour of the other components of the bipolar plate and the fuel
cell unit.
[0032] The specific electrical conductivity .sigma. of the
protective layer preferably ranges from approximately 0.01 S/cm to
approximately 200 S/cm.
[0033] The bipolar plate according to the invention is particularly
suitable for use in a high-temperature fuel cell, in particular an
SOFC (solid oxide fuel cell) with an operating temperature of at
least 600.degree. C., for example.
[0034] The present invention additionally relates to a process for
producing a protective layer on a bipolar plate for a fuel cell
unit.
[0035] A further object forming the basis of the invention is to
provide such a process, by means of which a protective layer is
produced that also has a favourable chromium retention power in
long-term operation and also meets the other requirements to be set
for a bipolar plate.
[0036] This object is achieved according to the invention by a
process for producing a protective layer on a bipolar plate for a
fuel cell unit, which comprises the following process steps: [0037]
applying a layer of a protective layer starting material to a
support layer of the bipolar plate, wherein the protective layer
starting material comprises Fe cations; [0038] generating a reduced
oxygen partial pressure; [0039] increasing the temperature to a
sintering temperature; [0040] subsequently increasing the oxygen
partial pressure; [0041] cooling the support layer and the
protective layer formed thereon.
[0042] By sintering the protective layer in a reducing atmosphere
the sintering temperature (usually from 900.degree. C. to
1100.degree. C.) can be reduced (in the range of approximately
750.degree. C. to approximately 800.degree. C.). Moreover, the
sintering time (usually 10 hours) can be shortened (to
approximately 3 hours at most, for example). As a result of this,
production costs can be saved and initial corrosive damages, in
particular as a result of a growth of a Cr.sub.2O.sub.3 layer at
elevated temperature and an associated lower electrical
conductivity, can be reduced. Moreover, this prevents the chromium
content in the steel material of the bipolar plate from decreasing
too significantly as a result of the growth of a Cr.sub.2O.sub.3
layer and the steel material thus losing its corrosion
resistance.
[0043] The reducing atmosphere is preferably selected such that at
least one of the metal oxides of the oxide system of the protective
layer is unstable, so that the associated metal cations have a
higher reactivity, whereas the reducing atmosphere is
simultaneously selected such that undesirable elements from the
starting material of the bipolar plate (in particular chromium) are
present in oxidic form. The oxidic form means a higher chemical
stability and thus a lower reactivity. As a result, a compaction of
the protective layer can occur during the sintering thereof without
chromium diffusing therein.
[0044] It is favourable if in the first sintering phase the
temperature and the oxygen partial pressure are selected such that
the state point defined by the sintering temperature and the
sintering oxygen partial pressure in the Ellingham diagram lies
above the stability limit of chromium oxide, but below the
stability limit of at least one metal oxide, the metal cation of
which is contained in the protective layer starting material.
[0045] It is particularly favourable if the support layer with the
starting material is not cooled between increasing the temperature
to sintering temperature and increasing the oxygen partial
pressure. In this way, the protective layer can be produced in a
single temperature cycle without intermediate cooling to room
temperature, which has a positive effect on the production costs
and the microstructure of the protective layer.
[0046] In a preferred configuration of the process according to the
invention, the starting material is applied to the support layer
using a wet-chemical method.
[0047] In this case, for example, the starting material can be
sprayed onto the support layer or also applied to the support layer
using the screen-printing process.
[0048] Further special configurations of the process according to
the invention are the subject of claims 16 to 23, the features of
which have already been explained above in association with the
special configurations of the bipolar plate according to the
invention.
[0049] Further features and advantages of the invention are the
subject of the following description and the diagrammatic
representation of exemplary embodiments.
[0050] In the drawings:
[0051] FIG. 1 is an Ellingham diagram, which shows the stability
limits of the oxides of chromium, iron, cobalt and manganese
according to Ellingham;
[0052] FIG. 2 shows a photomicrograph of a section through a
substrate of Crofer22 APU and a protective layer with the
composition MnCo.sub.1.9Fe.sub.0.1O.sub.4, which has been sintered
for three hours in a reducing atmosphere at a sintering temperature
of 800.degree. C.; and
[0053] FIG. 3 is a schematic view corresponding to FIG. 2 of a
section through a bipolar plate with a protective layer and an
intermediate layer arranged between the protective layer and a
starting material of the bipolar plate.
[0054] To produce the bipolar plate shown in sections in
longitudinal section in FIG. 2, the procedure is as follows:
[0055] A support layer is provided comprising a ferritic, chromium
oxide-forming special steel, e.g. Crofer22 APU special steel, which
has the following composition: 22.2% by weight Cr; 0.46% by weight
Mn; 0.06% by weight Ti; 0.07% by weight La; 0.002% by weight C;
0.02% by weight Al; 0.03% by weight Si; 0.004% by weight N; 0.02%
by weight Ni; the remainder iron.
[0056] In a first exemplary embodiment, in a wet spraying process a
suspension is sprayed onto this support layer that has the
following composition: 1 part by weight of a ceramic powder; 1.5
parts by weight of ethanol; 0.04 parts by weight of a dispersing
agent (e.g. Dolapix ET85); 0.1 parts by weight of a binding agent
(e.g. polyvinyl acetate, PVAC).
[0057] The ceramic powder for the suspension is produced as
follows:
[0058] Firstly, a quantity of three different metal oxides, e.g.
Mn.sub.2O.sub.3, Co.sub.3O.sub.4 and Fe.sub.2O.sub.3, are weighed
so that the numerical ratio of the respective metal cations (e.g.
Mn, Co, Fe) corresponds to the numerical ratio in the desired
composition of the protective layer to be produced (e.g. 1:1.9:0.1
in the composition MnCo.sub.1.9Fe.sub.0.1O.sub.4).
[0059] The weighed metal oxide powders are placed in a polyethylene
bottle together with ethanol and ZrO.sub.2 grinding balls (with an
average diameter of approximately 3 mm).
[0060] In this case, the weight ratio of powder:ethanol:grinding
balls amounts to approximately 1:2:3.
[0061] The polyethylene bottle is tightly sealed and rotated for 48
hours on a roller bench.
[0062] In this case, the rotational speed of the bottle amounts to
approximately 250 rpm.
[0063] After the said rotation time the grain size of the powder
should amount to d.sub.90=1 .mu.m.
[0064] If the specified grinding period of 48 hours is not
sufficient for this, the grinding time must be extended
accordingly.
[0065] A grain size of d.sub.90=1 .mu.m means that 90% by weight of
the particles of the ceramic powder have a grain size of 1 .mu.m at
most.
[0066] After the desired grain size of the ceramic powder has been
reached, the ZrO.sub.2 grinding balls are removed from the mixture
and the ceramic powder is dried.
[0067] The ceramic powder is then calcined at a temperature of
900.degree. C. with a holding time of six hours. In this case, the
powder is heated with a heating rate of 3 K/min. and cooled in an
unregulated manner after the holding time (natural cooling).
[0068] The ceramic powder obtained in this manner is mixed with
ethanol, dispersing agent and binder to form the suspension with
the aforementioned composition.
[0069] The suspension thus obtained is sprayed onto the support
layer through a spray nozzle in the wet spraying process.
[0070] In this case, the diameter of the nozzle orifice, with which
the suspension is atomised, amounts to approximately 0.5 mm.
[0071] The spraying pressure, with which the suspension is
transported to the nozzle, amounts to 0.3 bar, for example.
[0072] The spraying distance of the nozzle from the support layer
(substrate) amounts to 15 cm, for example.
[0073] The nozzle is moved across the support layer at a speed of
230 mm/s.
[0074] The layer of the protective layer starting material is
applied to the support layer in two to four coating cycles, i.e. by
coating each surface region of the support layer twice to
four-times.
[0075] Alternatively to the above-described wet spraying process, a
screen-printing process can also be used to apply the ceramic
powder to the support layer.
[0076] For such a screen-printing process a paste is produced,
which contains, for example, 50% by weight of the ceramic powder,
47% by weight of terpineol and 3% by weight of ethyl cellulose.
[0077] In this case the ceramic powder is produced in the same
manner as described above in association with the wet spraying
process.
[0078] To reduce the required grinding period, 2-4% by weight
(based on the weight of the ceramic powder) of a dispersing agent
(e.g. Dolapix ET85) can also be added to reach the specified grain
size.
[0079] The components of the paste are homogenised in a roller
frame.
[0080] The application of the paste of the protective layer
starting material to the support layer of the bipolar plate is then
performed by means of a screen-printing assembly known per se to
the person skilled in the art.
[0081] The support layer with the layer of the protective layer
starting material applied using the wet spraying process or
screen-printing process, for example, is firstly sintered in a
subsequent thermal treatment with a reduced oxygen partial
pressure.
[0082] For this, the support layer with the protective layer
starting material arranged thereon is placed in a sintering
oven.
[0083] The oxygen partial pressure is then reduced in the sintering
oven, e.g. by flushing using a mixture comprising an inert gas
(e.g. argon) and 4% mol of hydrogen, for example, which has been
previously moistened at a temperature of 25.degree. C., so that the
gas mixture has a water content of approximately 3% by weight.
[0084] After the oxygen partial pressure has been reduced in the
sintering oven in this manner, the oven is heated so that the
support layer with the starting material arranged thereon is heated
to a sintering temperature of at least approximately 750.degree.
C., preferably in the range of approximately 750.degree. C. to
800.degree. C. At this sintering temperature the support layer with
the starting material arranged thereon is held in a first sintering
phase for a sintering period of approximately 3 hours, for example,
as a result of which the layer of protective layer starting
material is sintered.
[0085] In this case, the reduction of the oxygen partial pressure
causes the original spine structure of the sintering additions to
break down, as a result of which the reactivity is increased and
the associated compaction process of the protective layer is
accelerated.
[0086] After the sintering period has ended, a changeover occurs to
an atmospheric oxygen partial pressure at uniform temperature in
order to restore the desired and chemically stable spinel structure
of the protective layer in a second sintering phase.
[0087] In this case, between the sintering process at reduced
oxygen partial pressure and the increase in the oxygen partial
pressure to an atmospheric oxygen partial pressure, the temperature
of the protective layer or the protective layer starting material
is not reduced to a temperature below 750.degree. C.
[0088] The reduced oxygen partial pressure during the sintering
process amounts to approximately 10.sup.-18, for example.
[0089] In the Ellingham diagram shown in FIG. 1 the sintering
temperature is indicated by the line 100 and the reduced oxygen
partial pressure during the first sintering phase is indicated by
the line 102.
[0090] The intersection point 104 of the lines 100 and 102
identifies the conditions in the first sintering phase.
[0091] In the Ellingham diagram this intersection point 104 lies
below the stability limit 106 of cobalt, but above the stability
limit 108 of iron, above the stability limit 110 of chromium and
above the stability limit 112 of manganese.
[0092] Therefore, in the temperature and oxygen partial pressure
conditions of the first sintering phase, the oxides of iron,
chromium and manganese are stable, whereas the oxides of cobalt are
unstable. Thus, under these conditions the cobalt cations have a
higher reactivity and therefore a higher sintering activity,
whereas undesirable elements from the steel of the support layer,
in particular chromium, are present in oxidic form. The oxidic form
means a higher chemical stability and thus a lower reactivity. As a
result, a compaction of the protective layer can occur in the first
sintering phase without chromium diffusing into the protective
layer.
[0093] As a result of the sintering of the protective layer in a
reducing atmosphere (because of the reduction of the oxygen partial
pressure) during the first sintering phase, the sintering
temperature, which usually amounts to 900.degree. C.-1100.degree.
C., can be reduced to approximately 750.degree. C. to 800.degree.
C. and the sintering time, which usually amounts to 10 hours, can
be decreased to approximately 3 hours. As a result of this, costs
can be saved in the production of the bipolar plate and initial
corrosive damages (degradation) of the support layer and the
protective layer, in particular too strong a growth of a chromium
oxide layer between the support layer and the protective layer at
elevated sintering temperature, can be reduced.
[0094] The bipolar plate, given the overall reference 114, obtained
after conclusion of the second sintering phase (under atmospheric
oxygen partial pressure) and comprising the support layer 116, the
protective layer 118 with the composition
MnCo.sub.1.9Fe.sub.0.1O.sub.4 and an intermediate layer 120, which
is formed between the support layer 116 and the protective layer
118 and contains cobalt-manganese-iron chromate, is shown in FIG. 3
in a purely schematic view in longitudinal section and in FIG. 2 in
a real microscopic view in longitudinal section.
[0095] Because of the comparatively low sintering temperature only
a little chromium diffuses out of the support layer 116 into the
intermediate layer 120, so that an undesirable reduction of the
chromium content in the steel of the support layer 116 is
prevented.
[0096] The microstructure of the protective layer 118 is improved
by the presence of iron cations in the starting material of the
protective layer 118. In particular, the protective layer 118 only
has few cracks and pores.
[0097] The iron cations therefore have a positive effect on the
sintering behaviour.
[0098] The improved microstructure of the protective layer 118, in
particular the reduced occurrence of cracks and pores, is an
indication that the protective layer is largely defect-free. Since
the chromium diffusion is based, inter alia, on the presence of
lattice defects, a lasting retention of chromium in the support
layer 116 and the intermediate layer 120 is assured by a protective
layer 118 that is as defect-free as possible.
[0099] As a result of the presence of iron cations in the
protective layer 118 its coefficient of thermal expansion .alpha.
is reduced and therefore it is better adapted to the coefficients
of thermal expansion of the steel material of the support layer 116
and to the coefficients of thermal expansion of other components of
the fuel cell unit, in which the bipolar plate 114 is to be used.
As a result, lower mechanical stresses occur in the fuel cell stack
with alternating temperature cycles.
[0100] Since the second sintering phase at atmospheric oxygen
partial pressure is conducted directly subsequent to the first
sintering phase at reduced oxygen partial pressure, no second
temperature cycle is necessary in the production of the bipolar
plate 116. This has a positive effect on the costs of the
production process and on the microstructure of the protective
layer 118 of the bipolar plate 114.
[0101] The coefficient of thermal expansion .alpha. of the
protective layer 118 produced in the above-described manner ranges
from approximately 1010.sup.-6K.sup.-1 to approximately
2010.sup.-6K.sup.-1.
[0102] The specific electrical conductivity .sigma. of the
protective layer 118 ranges from approximately 0.01 S/cm to
approximately 200 S/cm.
* * * * *